The present invention relates to improvements in apparatus for grinding the hard metal inserts or working tips of drill bits (percussive or rotary), tunnel boring machine cutters (TBM) and raised bore machine cutters (RBM) and more specifically, but not exclusively, for grinding the tungsten carbide cutting teeth or buttons of a drill bit or cutter.
In drilling operations the cutting teeth (buttons) on the drill bits or cutters become flattened (worn) after continued use. Regular maintenance of the drill bit or cutter by regrinding (sharpening) the buttons to restore them to substantially their original profile enhances the bit/cutter life, speeds up drilling and reduces drilling costs. Regrinding should be undertaken when the wear of the buttons is optimally one third to a maximum of one-half the button diameter.
Manufacturers have developed a range of different manual and semi-automatic grinding machines including hand held grinders, single arm and double arm self centering machines for setting up two or more bits to be ground, mobile machines for grinding on the road or in a workshop and grinders designed specifically for mounting on drill rigs, service vehicles or set up in the shop. The present invention is particularly applicable to mobile grinding apparatus of the type described in U.S. Pat. No. 5,193,312 and semi-automatic grinding machines as described in U.S. Pat. No. 5,070,654.
These types of machines utilize a grinding cup having the desired profile rotated at high speed to grind the carbide button and the face of the bit/cutter surrounding the base of the button to restore the button to substantially its original profile for effective drilling. When grinding buttons, the centering aspects of the grinding machine tend to center the grinding machine over the highest point on the button. On buttons where wear is uneven, typically gauge buttons, this may result in regrinding the button off center from its longitudinal axis.
The conventional grinder designs switch between grinding pressure and balance pressure to achieve the desired effect. This, for example, does not allow for a grinding pressure equal to zero. In conventional grinder designs, the minimum grinding pressure is equivalent to the weight of the arm or lever section and the components attached to it. This may, for example, result in grinding pressure that is too great in relation to what is optimum for the size, type and profile of the button being sharpened.
The gauge buttons are mounted in the bit at an angle relative to the face of the bit. In order to properly regrind a worn gauge button the bit must be tilted to correspond to the angle at which the gauge buttons are mounted in the bit. In order to regrind all the buttons on the bit, the operator must continually, tilt and rotate the bit as he proceeds from button to button and/or bit to bit.